1. Field of the Invention
This invention relates to an aerodynamic control system for structural stability augmentation of a forward swept wing aircraft.
2. Statement of Prior Art
Forward swept wings of metal are limited in their use due to the static stability phenomenon of divergence at high speed flight conditions. Making the wing out of composite materials offers relief from this phenomenon. However, forward swept wings made from composites are still quite flexible when installed on aircraft. This has led to the observation of a newly discovered dynamic stability phenomenon called rigid body (whole vehicle) structural mode coupling. This occurs as a result of coupling between properly phased wing bending and the aircraft rigid body pitching mode when the frequencies of the two modes are close together. The consequences of rigid body structural mode coupling (also known as rigid body/wing bending flutter) are structural dynamic instability that can be critical with certain aircraft configurations, a degradation in aircraft handling and ride qualities, and increased wing design loads for gusts.
Stability augmentation systems for augmenting whole vehicle motion response are well known in the art. One such system is disclosed in U.S. Pat. No. 4,171,115 to Osder. Another such system is disclosed in U.S. Pat. No. 3,819,135 to Foxworthy, et al. An application of such a system to a forward swept wing aircraft is disclosed in U.S. Pat. No. 2,420,932 to Cornelius.
Augmentation systems have also been used on flexible aircraft to control structural motion and provide stability. Examples of such systems are disclosed in U.S. Pat. Nos. 3,412,961 and 3,279,725. U.S. Pat. No. 3,902,686 discloses a structural mode control system having sensing elements to operate control force application devices such as aerodynamic control vanes to obtain structural damping. Sensing elements are located near control force application points. In U.S. Pat. No. 3,347,498, a system is disclosed which utilizes accelerometers employed on the wing and fuselage to sense structural accelerations of the wing and to operate wing control surfaces to reduce wing structural stress. Rigid body motion and structural motion are separated by appropriate placement of sensors. A flutter suppression system is disclosed in U.S. Pat. No. 3,734,432. Here one or more pairs of leading and trailing edge control surfaces are operated by a stability augmentation system to suppress flutter. De-stabilizing effects on the rigid body mode are opposed by additional leading and trailing edge control pairs on the horizonal tail or canard.